For the selective removal of Cs + from medium-active waste solutions, some new extraction agents were developed, based on the crown ether dibenzo-21-crown-7 combined with well-known reagents selective for Cs + . The extractive efficiency of the crown ether and of these reagents (which by themselves are not stable in MAW solutions) is synergistically enhanced on combination. The solution of the crown ether alone shows no satisfactory extraction coefficients.Among the investigated combinations of extraction agents representing a new kind of liquid ion-exchangers, those containing a 12-heteropolyacid or hexachloroantimony(V) acid, or the corresponding alkali salts, dissolved in 1,1,2,2-tetrachloroethane or nitrobenzene, meet all the requirements for the application in continuously-working extraction plants: high selectivity for Cs + , extraction coefficients in the range of some 10 to some 100, high chemical stability to strongly nitric solutions, very small bleeding, i.e. practically no loss of the extraction agent, easy regeneration.